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Well I'm a little confused. I was under the impression that to determine the speed of a bike you take the number of chaining's and multiply that by the number of cogs. But I have a bike that has three chainrings and five cogs, which to me is a fifteen speed bike. But all the information on the bike says it is a eighteen speed. The owner of the bike is the original owner and he has never had any changes done to this bike. Am I missing something? Or is this bike labeled incorrectly. It is a Thruster ATB bike. Not sure who makes this brand as I can't seem to find any information on it.

Count the chain rings again to make sure or instead of a 6 speed in the rear someone put a 5 speed on. The high and low gear is probably the same no matter what so it makes little or no difference. Roger

if the bike was purchased used
then it is likely the previous owner swapped parts
and it wound up with a 5 speed freewheel

the other possibility
is that you miscounted the cogs on the rear
it is an easy mistake to make
remember when counting that there is a cog under thd chain

the problem with having a different number of cogs
than originally came on the bike
is that the indexing on the shifters will not work weell
and the derailleur may need to be adjusted to work properly with the freewheel

And besides the real count of cogs x rings there is the issue of the actual ratios the various combos provide. If you divide the ring by the cog you get the ratio of a particular combo. Going through all (15 combos for the OP) ratios can show that some combos are nearly the same ratio as others. So the 15 speed (or different ring/cog combos) might more realistically be an 11 speed by practical gear differences. Andy.

And besides the real count of cogs x rings there is the issue of the actual ratios the various combos provide. If you divide the ring by the cog you get the ratio of a particular combo. Going through all (15 combos for the OP) ratios can show that some combos are nearly the same ratio as others. So the 15 speed (or different ring/cog combos) might more realistically be an 11 speed by practical gear differences. Andy.

+1

The number of usefully different ratios is usually about 2/3 of the number of available combinations.

Now, exactly how much difference there needs to be considered usefully different can probably be endlessly debated, if anyone feels so inclined. And it will differ between setups anyhow. Me, I'm happy with the 2/3 ballpark.

Well I guess that someone must have messed up when they put the 18 speed decal on this bike. The original owner says that he never changed anything on this bike and to have three chainrings on the front and five cogs on the rear is not an eighteen speed. Oh well something new to experience.